P-v-fe catalyst modified with chromium for production of maleic anhydride from saturated aliphatic hydrocarbons

ABSTRACT

Maleic anhydride is produced by oxidation of saturated aliphatic hydrocarbons in the presence of a catalyst comprising phosphorus, vanadium, iron, oxygen and chromium.

United States Patent Raffelson et a].

1 51 Feb. 18, 1975 P-V-FE CATALYST MODIFIED WITH CHROMIUM FOR PRODUCTION OF MALEIC ANHYDRIDE FROM SATURATED ALIPHATIC HYDROCARBONS Inventors: Harold Raffelson; Michael Suda,

both of St. Louis, Mo.

Assignee: Monsanto Company, St. Louis, Mo.

Filed: Dec. 27, 1971 Appl. No: 212,792

us. ca. 260/3468 A, 252/437 Int. Cl. C07c 57/14 Field of Search 260/3468 A References Cited UNITED STATES PATENTS Rylander et al. 260/68315 ll/1964 Kerr 260/3468 A 3,156,706 11/1964 Kerr 260/3468 3,293,268 12/1966 Bergman et a1 260/3468 3,478,063 11/1969 Friedrichsen et a1, 260/3468 A FOREIGN PATENTS OR APPLICATIONS OTHER PUBLICATIONS Chemical Abst., 1971 v01. 75, 77488.

Germany Primary Examiner-Henry R. Jiles Assistant Examiner-Bernard Dentz Attorney, Agent, or FirmN. E. Willis; J. E. Maurer; F. D. Shearin 6 Claims, N0 Drawings P-V-FE CATALYST MODIFIED WITH CHROMIUM FOR PRODUCTION OF MALEIC ANI-IYDRIDE FROM SATURATED ALIPIIATIC IIYDROCARBONS This invention relates to the catalytic production of maleic anhydride. More particularly, the invention relates to a process for the production of maleic anhydride by the oxidation of a saturated aliphatic hydrocarbon having from 4 to carbon atoms, the process being carried out in the presence of a surprisingly advantageous catalyst.

Production of maleic anhydride by the catalytic oxidation of benzene and butene is well known. The principal method currently employed for manufacturing maleic anhydride is by the air oxidation of benzene in the presence of certain heavy metal oxide catalysts.

Comparatively little attention, however, has been given to the use of saturated aliphatic hydrocarbons,

e.g., butane, as feedstocks for the preparation of maleic anhydride. This is due in part to the fact that butane is more difficult to oxidize than benzene or butene, often resulting in low conversions to'maleic anhydride and an uneconomical process. Although catalysts for the oxidation of saturated aliphatic hydrocarbons are known, the yields of the desired maleic anhydride product over the known catalysts are, in most cases, insufficiently high to make such feedstocks competitive with benzene. Efficient conversion of the saturated aliphatic hydrocarbon to maleic anhydride, together with longer catalyst life, is desirable.

Accordingly, it is an object of the present invention to provide a process for obtaining improved yields and selectivity of maleic anhydride together with improvements in catalyst stability.

A further object of the present invention is to provide a process for the catalytic oxidation of saturated aliphatic hydrocarbons to maleic anhydride in improved yields and improved selectivities.

Still a further object of the present invention is to provide an improved type of catalyst for carrying out the catalytic oxidation of saturated aliphatic hydrocarbons to maleic anhydride.

These and other objects are accomplished according to the present invention by oxidizing a saturatedaliphatic hydrocarbon having from 4 to 10 carbon atoms, in the presence of a catalyst comprising phosphorus, vanadium, iron and oxygen, together with chromium as a catalyst modifier. When the phosphorus, vanadium and iron are present in defined proportions, along with the chromium modifier, unexpected improvements are achieved in yields of maleic anhydride from saturated aliphatic hydrocarbon feedstocks, as compared-to results obtained with similar phosphorus-vanadium-ironoxygen catalyst compositions containing no chromium. Improvements in catalyst stability have also been observed.

Optimum results are obtained according to the present invention when the atomic ratio of phosphorus to vanadium in the active component of the. catalyst is between about 4:1 and about 9:1 with the atomic ratio of iron to vanadium between about 1:1 and about 4:1. Thus, it is preferred to employ catalysts having proportions of the three basic metals within the aforementioned ranges. The chromium, introduced as a modibly from about 5 atomic percent to about 12 atomic percent. By atomic percent herein is meant the fractional part (expressed as a percentage of the atomic sum) of the metals present in the active component of the catalyst. An outstanding catalyst composition within the scope of the present invention contained 62.0 atomic percent phosphorus, 9.0 atomic percent vanadium, 20.0 atomic percent iron and 9.0 atomic percent chromium, representing a phosphorus to vanadium atomic ratio of 6.89:1 and an iron to vanadium atomic ratio of 2.22:1.

Superior results can still be realized, however, when the atomic ratio of phosphorus to vanadium is maintained between about 1:1 and about l5:l, while the atomic ratio of iron to vanadium is between about 0.211 and about 6:1, again with a stabilizing amount of chromium present.

The catalysts taught by the present invention can be prepared in a variety of ways readily apparent to those skilled in the catalyst art. However, it is preferred to prepare them from an aqueous slurry or solution of phosphorus, vanadium, iron and chromium compounds and subsequently heat this aqueous mixture to approximately 90C. Thereupon, the batch is cooled and ammonium hydroxide is added to obtain the desired pH, the ammonium hydroxide further affording precipitation of the catalyst composition from solution. The catalyst composition can then be deposited onto a support or carrier although it has been found that an unsupported catalyst (without carrier) made, for example, by extruding the catalyst material, usually affords higher yields in the process of the present invention. The catalyst may also be prepared by precipitating the vanadium, phosphorus, iron and chromium compounds, either with or without a carrier, from a colloidal dispersion of the ingredients in an inert liquid. The catalyst may also be prepared by heating and mixing anhydrous forms of phosphoric acid with the vanadium compounds,-the iron compounds and the chromium compounds. The catalysts may be used as either fluid bed or fixed bed catalysts. In any ofthe methods ofpreparation, heat may be applied to accelerate the formation of the catalyst complex. Although certain methods of catalyst preparation are preferred, any method may be used which results in the formation of the catalyst complex containing the specified ratios of vanadium, phosphorus and iron, together with the desired amount of chromium modifier.

Suitable phosphorus compounds that may be employed in the preparation of catalysts herein include mono-, diand tri-ammonium phosphates, metaphosphoric acid, triphosphoric acid, pyrophosphoric acid, orthophosphoric acid, phosphorus pentoxide, phosphorus oxyiodide, ethyl phosphate, methyl phosphate,

' phosphorus pentachloride, phosphorus trichloride,

phosphorus oxybromide, iron phosphates and the like.

Suitable vanadium compounds useful as starting materials are compounds such as vanadium pentoxide, ammonium metavanadate, vanadium trioxide, vanadyl chloride, vanadyl dichloride, vanadyl trichloride, vanadium sulfate, vanadium phosphate, vanadium tribromide, vanadyl formate, vanadyl oxalate, metavanadic acid, pyrovanadic acid, vanadyl sulfate and the like.

Various iron compounds, both ferric and ferrous, such as ferrous sulfide, ferric and ferrous halides, ferric and ferrous sulfate, ferric nitrate, ferrous oxalates, fer- 3 rous citrate, ferrous phosphates and the like can be used as the source of iron The chromium modifier may be added as the element per se or as a compound thereof such as the oxide, hydroxide, carbonate, phosphate, nitrate, sulfate and inorganic or organic salts or acids of chromium.

Thus, any vanadium, phosphorus, iron and chromium compounds may be used as starting materials which, when the compounds are combined and heated to dryness in air at a temperature of, for example, l 10C., followed by calcining at, for example, 500C., will leave as a deposit a catalyst complex having relative proportions within the described ranges. Calcining is most conveniently accomplished by exposing the catalyst composition to a stream of air at a temperature between about 400C. and 600C., but may also be accomplished with an inert gas or oxygen.

Although the catalysts useful in the present invention produce superior results when separately formed, such as by extrusion, and used as pellets without a supporting carrier, it is sometimes desirable to deposit the catalyst material onto a carrier. The carrier may be added to the catalyst complex containing phosphorus, vanadium, iron, oxygen and chromium, or the catalyst solution may be poured onto the carrier. Also, the carrier may be present during the entire course of reactions to provide the desired phosphorus-vanadium-ironoxygen-chromium complex.

The support or carrier for the phosphorus-vanadiumiron-oxygen-chromium complex, if any, should preferably be inert to the depositing solution containing the complex and should be inert under the catalytic oxidation conditions. The carrier provides not only the required surface for the catalyst, but gives physical strength and stability to the catalyst material. The carrier or support preferably has a low surface area, as

usually measured, from about 0.001 to about-5 square meters per gram. Carriers much smaller than to 12 mesh normally cause an undesirable pressure drop in the reactor, unless the catalysts are being used in a fluid bed apparatus. Any of the conventional inert carriers may be used in this capacity, including silica gel, silica alumina, silica, carborundum, kieselguhr, boron phosphate, alundum, fullers earth, pumice, silicon carbide and asbestos.

The amount of catalyst complex (active catalytic component) deposited on the carrier should be sufficient to substantially coat the surface ofthe carrier. For a fixed bed reactor installation, the final size of the catalyst coated on a carrier will preferably be from about 2 /zto about 10 mesh size. The carriers may be of a variety of shapes, the preferred shape being cylindrical or spherical. The size of the catalyst particles used in fluidized bed reactors is quite small, however, usually varying from about 10 to about 150 microns, and in such systems the catalyst normally will not be provided with a carrier but will be formed into the desired particle size after drying from solution.

Inert diluents such as silica may be present in the active portion of the catalyst, but'the combined weight of the essential active ingredients of phosphorus, vanadium, iron, oxygen and chromium should preferably consist essentially of at least about 50 weight percent of the composition which is coated on the carrier, if any, and preferably these components are at least about 75 weight percent of the composition coated on the carrier, and more preferably at least about 95 4 weight percentflf desired, any remainder other than the atoms of phosphorus, vanadium, iron, oxygen and chromium may be essentially inert non-catalytic ingredients intimately combined with the phosphorus, vanadium, iron, oxygen and chromium as a part of the coating on the carrier.

In accordance with the present invention, various saturated aliphatic hydrocarbons having from 4 to 10 carbon atoms are oxidized over the phosphorus-vanadiumiron-oxygen-chromium catalyst to produce maleic anhydride. Since n-butane is readily available in good purity at a relatively low price, as compared to other saturated aliphatic hydrocarbons of the indicated class, it is preferred to employ n-butane in the practice of the present invention. Accordingly, principal reference is made in the present specification to the preparation of maleic anhydride by the catalytic oxidation of nbutane. It will be understood, however, that the oxidation of other saturated aliphatic hydrocarbons having from 4 to 10 carbon atoms is within the scope of the present invention.

The process of the present invention is most conveniently carried out by passing a stream of oxygen or oxygen-containing gas, usually air, containing the saturated aliphatic hydrocarbon to be oxidized, over a bed of the catalyst contained in any conventional fixed bed converter tube. The oxidation reaction takesplace at temperatures of from about 400 to 600C., preferably from about 450 to 550C., under pressures ranging from about 0.5 to 10 atmospheres, desirably about 1 to 5 atmospheres. In the case of n-butane, optimum yields of maleic anhydride are achieved when the n-butane is admixed with air in the proportion of from about 0.5 to 10 mole percent, preferably from about 1 to about 4 mole percent, of the total oxidation mixture, and passed over the catalyst while maintaining a space velocity between about 200 and 7,000 reciprocal hours, preferably in the case of fixed bed reactors, from about 1,000 to 3,000 reciprocal hours. Space velocity is defined herein as the volume of gaseous mixture at C. and 760 mm. Hg pressure per volume of empty converter tube in the catalyst zone per hour.

A variety of reactors will be found to be useful and multiple tube heat exchanger type reactors are quite satisfactory. The tubes of such reactors may vary in diameter from about A inch to about 3 inches, and the length may be varied from about 3 to about 10 or more feet. Since the oxidation of n-butane or other saturated aliphatic hydrocarbons within the scope of the present invention is an exothermic reaction, it is often necessary to remove the heat of reaction in order to maintain the reaction temperature within the preferred range of from about 450 to 550C. It is thus desirable to have the surface of the reactors at a relatively constant temperature and some medium to conduct heat from the reactors is usually necessary to aid temperature control. Such media may be mercury, molten lead and the like, but is had been found that eutectic salt baths are completely satisfactory. One such salt bath is a sodium nitrate-sodium nitrite-potassium nitrate eutectic constant temperature mixture. An additional method of temperature control is to use, a metal block reactor whereby the metal surrounding the tube acts as a temperature regulating body. The heat exchange medium may be kept at the proper temperature by heat exchangers and the like. The reactor or reaction tubes may be fabricated of iron, stainless steel, carbon-steel,

glass tubes, and the like. Normally, the reactors contain a preheat zone of an inert material such as A1 inch alundum pellets, inert ceramic balls, nickel balls or chips, and the like.

The catalytic oxidation reaction may be conducted at atmospheric, superatmospheric or subatmospheric pressure, although superatmospheric pressures are generally employed.

Maleic anhydride produced according to the present invention may be recovered in a number of ways well known to those skilled in the art. For example, the recovery may be by direct condensation or by adsorption in suitable media, with subsequent separation and purification of the maleic anhydride.

Other saturated aliphatic hydrocarbon feedstocks within the scope of the present invention, in addition to butane, are the pentanes, hexanes, heptanes,'octanes, nonanes and decanes, or mixtures of any of these with or without butane. The aforementioned feedstocks are not necessarily pure substances but maybe technical hydrocarbon fractions containing small amounts, for example, 0.1 to percent by weight of olefinically unsaturated hydrocarbons containing more than 4, for example, 5 or 6, carbon atoms, or containing other hydrocarbon fractions.

The present invention will be more readily understood by reference to the following detailed description of preferred embodiments thereof. In each of the examples set forth below, percentage yields are expressed on the basis of the number of pounds of pure maleic anhydride produced per pound of n-butane fed to the reactor. Also included is the percentage selectivity for each example which is expressed on the basis of the number of pounds of maleic anhydride produced per pound of butane reacted. The temperatures quoted in the tables are reaction temperatures which are considered as equivalent to catalyst temperatures.

To illustrate the improved performance afforded by the presence of chromium in the catalyst taughtby the present invention, Example 1 below is representative of the results achieved with no chromium present. Example 2 and the subsequent examples are illustrative of results obtained with various modified catalysts within the instant scope.

EXAMPLE 1 This example illustrates the preparation of maleic anhydride by the oxidation of n-butane using a catalyst wherein the catalytic component comprised only phosphorus, vanadium, iron and oxygen.

The catalyst was prepared as follows:

4.32 grams of ammonium metavanadate -were dissolved in 162 ml. of water with heating and stirring at 80C. To the hot ammonium metavanadate solution were added 108 ml. of water, 23.89 grams of ferric nitrate, 20.0 grams of 85 percent orthophosphoric acid and 2.49 ml. of nitric acid. The batch was heated to 90C. and held, with stirring, for about 10 minutes. The batch was then cooled below 60C. and 23.14 ml. of ammonium hydroxide were added to obtain a pH of approximately 8. A silica alumina carrier (100 grams) having a screen size of 3 to 5 mesh was added and the batch was heated on a steam bath to 90C. with stirring. The catalyst and carrier was then dried in a 110C. oven for approximately hours, followed by calcining at 500C. for about-4 hours in a tube with air passing over the batch. After screening with a number 6 mesh screen, 80.9 grams of supported catalyst were recovered which had a calculated compositiontin the catalytic component) of 13.71 atomic percent vanadium, 64.35 atomic percent phosphorus and 21.94 atomic percent iron.

The resulting catalyst was charged (in a volume of 50 ml.) to a 1 inch 1.D. fixed bed glass reactor tube and heated to about 500C. A gaseous mixture, predominantly air, containing approximately 1.6 mole percent of n-butane was passed through the catalyst bed while maintaining a nominal space velocity of 1200 reciprocal hours. The maleic anhydride thus prepared was removed from the gaseous mixture by scrubbing the reactor effluent gases in water. In successive runs conducted under slightly varying operating conditions, the following results were obtained.

This example illustrates the preparation of maleic anhydride by the oxidation of n-butane using a catalyst within the present invention wherein the active catalytic component comprises phosphorus, vanadium, iron and oxygen modified with chromium. The catalyst was prepared as follows:

102.35 grams of ferric nitrate were dissolved in 400 ml. of water with heating and stirring. To the hot ferric nitrate solution were added 45.6 grams of chromium nitrate, 90.55 grams of percent orthophosphoric acid and 3.3 ml. of nitric acid. With the batch temperature at 70C., 13.35 grams of ammonium metavanadate were added whereupon the batch was heated to C. and held, with stirring, for 15 minutes. The batch was then cooled to 50C. and 260 m1. of ammonium hydroxide were added to obtain a pH of 10. A silicaalumiria carrier (500grams) having a screen size of 3 to 5 mesh was added after the batch had been heated on a steam bath to approximately 90C. with stirring for 30 minutes. The catalyst and carrier blend was then dried in a l 10C. oven for 4 hours, screened with a Number 6 mesh screen, then calcined at 500C. After further screening, 525.5 g. ofsupported catalyst were recovered which had a calculated metal composition (in the catalytic component) of 62.0 atomic percent phosphorus, 9.0 atomic percent vanadium, 20.0 atomic percent iron and 9.0 atomic percent chromium.

The resulting catalyst was charged to a 2 ft. long fixed bed steel reactor tube having a 0.824 1.D., and the catalyst was heated to about 500C. A gaseous mixture, predominantly air, containing approximately 1.4 mole percent of n-butane was passed through the catalyst bed while maintaining a nominal space velocity of 1,300 reciprocal hourssThe maleic anhydride thus pretions over a period of 181 hours, the following results were obtained:

An extended test on the catalyst of Example 2 revealed good stability after 2300 hours of operation at 475C. catalyst temperature, 1.4 mole percent of nbutane and 1,300 reciprocal hours space velocity.

EXAMPLE 3 This example illustrates the preparation of maleic anhydride by the oxidation of n-butane using another phosphorus-vanadium-iron-oxygen-chromium catalyst within the scope of the present invention. The catalyst was prepared as follows:

23.63 grams of ferric nitrate were dissolved in 125 ml. of water with heating and stirring. To the hot ferric nitrate solution were added 6.10 grams of chromium nitrate, 18.76 grams of 85 percent orthophosphoric acid and 1.13 ml. of nitric acid. The batch was heated with stirring to 90C. followed by the addition of 2.08 grams of ammonium metavanadate. After further heating and stirring at 90C. for approximately 15 minutes, the batch was cooled to about 50C. and 50 ml. of ammonium hydroxide were added to achieve a pH of 10.

The batch was reheated to 90C. and held, with stirring,

for 45 minutes. A silica alumina carrier (100 grams) having a screen size of 3 to 5 mesh was then-added to the batch. The catalyst and carrier blend were ovendried at 110C. for several hours, screened with a Numher 6 mesh screen, then calcined at 500C. After further screening, 82.0 grams of supported catalyst were recovered which had a calculated metal composition in the catalytic component of 64.0 atomic percent phosphorus, 7.0 atomic percent vanadium, 23.0 atomic percent iron and 6.0 atomic percent chromium.

The resulting catalyst was charged to a 2 ft. long fixed bed glass reactor tube having a 1 inch 1.D., and the catalyst was heated to about 500C. The gaseous mixture, predominantly air, containing approximately 1.5 mole percent of n-butane was passed through the catalyst bed while maintaining a nominal space velocity of 1250 reciprocal hours. The maleic anhydridethus prepared was removed from the gaseous stream by scrubbing the reactor effluent gases in water. In successive runs conducted under slightly varying operating conditions, the following results were obtained:

EXAMPLE 4 Still another phosphorus, vanadium, iron, oxygen and chromium catalyst was prepared and employed in the production of maleic anhydride from n-butane, the catalyst being prepared as follows:

2.13 grams of ammonium metavanadate were dissolved in 125 ml. of water with heating and stirring at C. To the hot ammonium metavanadate solution were added 13.78 grams of 85 percent orthophosphoric acid, 13.88 grams of ferric nitrate, 7.30 grams of chromium nitrate and 1.27 grams of ferric phosphate. The batch was heated to 90C. and held, with stirring, for 1 hour followed by cooling to 50C. and the addition of 23.9 ml. of ammonium hydroxide to obtain a pH of 8. The batch was then heated to 90C. for 2 hours and -l0 minutes and grams of silica alumina carrier having a screen size of 3 to 5 mesh were added to the batch followed by stirring for 30 minutes. The catalyst and carrier blend was then dried in a 1 10C. oven for 4 hours, screened with a Number 6 mesh screen, then calcined at 500C. After further screening, 90.5 grams of supported catalyst were recovered which had a calculated metal composition in the catalytic component of 62.0 atomic percent phosphorus, 9.0 atomic percent vanadium, 20.0 atomic percent iron and 9.0 atomic percent chromium. The resulting catalyst was charged to a 2 ft. long fixed bed glass reactor tube having a 1 inch 1.D., andthe catalyst was heated to about 500C. A gaseous mixture, predominantly air, containing approximately 1.6 mole percent of n-butane was passed through the catalyst bed while maintaining a nominalspace velocity of 1030 reciprocal hours. The maleic anhydride product was removed from the gaseous stream by scrubbing the reactor effluent gases in water. The following results were obtained from the successive runs with this catalyst:

The following example illustrates the preparation of maleic anhydride from n-butane with yet another phosphorus-vanadium-iron-oxygen-chromium containing catalyst: I

2.14 grams of ammonium metavanadate were dissolved in 350 ml. of water in a 1 liter round bottomed flask equipped with a stirrer and thermometer. The solution was heated to 75C. followed by the addition of 13.81 grams of percent orthophosphoric acid, 1.28 grams of ferric phosphate, 9.80 grams of ferric nitrate and 11.38 grams of chromium nitrate. The batch was then heated to C. for one hour after which it was cooled to below 50C. for the addition of 27 ml. of ammonium hydroxide which adjusted the pH to 8. The batch was then stripped for 3 hours to remove excess water after which the resulting slurry was poured into a casserole over silica alumina carrier pellets having a screen size of 3 to 5 mesh. The catalyst and carrier blend was dried on a steam bath and then placed in a 110C. oven for 4 hours. After screening with a Number 6 screen, the supported catalyst was calcined at 500C, resulting in 88.3 grams of catalyst having a calculated metal composition in the catalytic component of 62.0 atomic percent phosphorus, 9.0 atomic percent vanadium, 15.0 atomic. percent iron and 14.0 atomic percent chromium.

The resulting catalyst was charged to a 2 ft. long fixed bed glass reactor tube having a 1 inch [.D. and the catalyst was heated to about 500C. A gaseous mixture, predominantly air, containing approximately 1.45 mole percent of n-butane was passed through the catalyst bed while maintaining a nominal space velocity of 1230 reciprocal hours. The maleic anhydride product was removed by water-scrubbing of the effluent gases. The following results were obtained under slightly varying operating conditions:

This example illustrates the preparation of maleic anhydride from n-butane with an unsupported phosphorus-vanadium-iron-oxygen-chromium catalyst made by extrusion.

To a 3 liter flask containing 1400 cc. of water were added 31.95 grams of ammonium metavanadate, 217.20 grams of 85 percent orthophosphoric acid, 245.70 grams of ferric nitrate, 109.50 grams of chromium nitrate and 7.5 ml. of nitric acid. The batch was heated to 90C. for 1 hour after which it was cooled below 50C. and neutralized with 435 ml. of ammonium hydroxide to achieve a pH of 8. Water was then distilled off until a thick slurry was formed whereupon three-sixteenth inch unsupported and calcined pellets were obtained by extrusion. This unsupported catalyst had a calculated metal composition of 62.0 atomic percent phosphorus. 9.0 atomic percent vanadium, 20.0 atomic percent iron and 9.0 atomic percent chromium.

The resulting catalyst was charged to a 2 ft. long fixed bed glass reactor tube having a 1 inch ID. and the catalyst was heated to about 450C. A gaseous mixture, predominantly air, containing approximately 1.45 mole percent of n-butane was passed through the catalyst bed while maintaining a nominal space velocity of 1200 reciprocal hours. The maleic anhydride product was removed by water-scrubbing of the effluent gases. The following results were obtained from successive runs with this catalyst:

TABLE Vl-Continued Space Tem n-Butane Feed Velocity Yield Selectivity C. mole '7! of Feedgas Hours 7: 72

By comparing the results of Examples 2 through 5 above with the results of Example 1, it can readily be observed that the presence of chromium in the phosphorus-vanadiumiron-oxygen catalyst contributes to the improved process for the production of maleic anhydride from saturated aliphatic hydrocarbons. It has thus been found that higher operating temperatures, e.g.. 520C, appear to cause the oxidation catalyst to lose stability faster than at temperatures such as 470C. The reduced stability is characterized by lower yields of maleic anhydride. By reference to Table l, for example. it is seen that the yields of maleic anhydride diminished at higher oxidation temperatures, there being no chromium present in the catalyst complex. This yield reduction trend at increased temperatures was much less pronounced with the chromium-modified catalyst as illustrated by Tables 11 through V above. Table VI illustrates that excellent yields can be achieved at lower oxidation temperatures with unsupported catalysts of the present invention.

, The surprising and unexpected nature of the present invention is illustrated by the discovery that chromium is singularly outstanding as a modifier for the phosphorus-vanadium-iron-oxygen catalyst for oxidation of saturated aliphatic hydrocarbons to maleic anhydride. It is not completely understood why the presence of chromium contributes to superior catalyst performance.

A further illustration of the efficacy of the phosphorus-vanadium-iron-oxygen-chromium catalyst hereinbefore described is the essentially perfect material balance which can be demonstrated. Thus, when computing the percentage of n-butane which is converted to maleic anhydride, along with the percentage of feed gas which goes to carbon monoxide and carbon dioxide, it has consistently been found that no compounds remain unaccounted for in the process.

Still another indication of the superiority of the present process is the avoidance of tar formation in the reaction equipment and the essentially water white appearance of the maleic anhydride liquor produced.

While this invention has been described with reference to various specific examples and embodiments, it is understood that the invention is not limited thereto and that it can be variously practiced within the scope of the following claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for producing maleic anhydride by the catalytic oxidation of a saturated aliphatic hydrocarbon having from 4 to 10 carbon atoms, wherein a stream of oxygen or oxygen-containing gas containing from about 0.5 to about 10 mole percent hydrocarbon is passed over the catalyst at a space velocity of between about 200 and about 7,000 reciprocal hours at a temperature of from about 400 to about 600C. under a pressure ranging from about 0.5 to 10 atmospheres, the

improvement which comprises conducting said oxidation in the presence of a catalyst comprising phosphorus, vanadium, iron, oxygen and chromium, wherein the atomic ratio of phosphorus to vanadium is from about 1:1 to about l5:l, the atomic ratio of iron to vanadium is from about 0.2:l to about 6:l, and the chromium is present in the range of from about 2 to about atomic percent of the active metals.

2. A method of claim 1 wherein the atomic ratio of phosphorus to vanadium in the catalyst is from about 4:1 to about 9:1, the atomic ratio of iron to vanadium is from about izl to about 4:1, and the chromium is present in the range of from about 5 to about 12 atomic percent of the active metals.

3. A method of claim 1 wherein the hydrocarbon comprises from about 1 mole percent to about 4 mole percent of the oxidation mixture.

4. A method of claim 1 wherein the hydrocarbon is n-butane.

5. In a process of claim 2 wherein the catalyst comprises from about 6 to about 9 atomic percent chromium.

6. In a process of claim 2 wherein the catalyst comprises about 9 atomic percent chromium. 

1. IN A PROCESS FOR PRODUCING MALEIC ANHYDRIDE BY THE CATALYTIC OXIDATION OF A SATURATED ALIPHATIC HYDROCARBON HAVING FROM 4 TO 10 CARBON ATOMS, WHEREIN A STREAM OF OXYGEN OR OXYGEN-CONTAINING GAS CONTAINING FROM ABOUT 0.5 TO ABOUT 10 MOLE PERCENT HYDROCARBON IS PASSED OVER THE CATALYST AT A SPACE VELOCITY OF BETWEEN ABOUT 200 AND ABOUT 7,000 RECIPROCAL HOURS AT A TEMPERATURE OF FROM ABOUT 400* TO ABOUT 600*C. UNDER A PRESSURE RANGING FROM ABOUT 0.5 TO 10 ATMOSPHERES, THE IMPROVEMENT WHICH COMPRISES CONDUCTING SAID OXIDATION IN THE PRESENCE OF A CATALYST COMPRISING PHOSPHORUS, VANADIUM, IRON, OXYGEN AND CHROMIUM, WHEREIN THE ATOMIC RATIO OF PHOSPHORUS TO VANADIUM IS FROM ABOUT 1:1 TO ABOUT 15:1 THE ATOMIC RATIO OF IRON TO VANADIUM IS FROM ABOUT 0.2:1 TO ABOUT 6:1, AND THE CHROMIUM IS PRESENT IN THE RANGE OF FROM ABOUT 2 TO ABOUT 20 ATOMIC PERCENT OF THE ACTIVE METALS.
 2. A method of claim 1 wherein the atomic ratio of phosphorus to vanadium in the catalyst is from about 4:1 to about 9:1, the atomic ratio of iron to vanadium is from about 1:1 to about 4:1, and the chromium is present in the range of from about 5 to about 12 atomic percent of the active metals.
 3. A method of claim 1 wherein the hydrocarbon comprises from about 1 mole percent to about 4 mole percent of the oxidation mixture.
 4. A method of claim 1 wherein the hydrocarbon is n-butane.
 5. In a process of claim 2 wherein the catalyst comprises from about 6 to about 9 atomic percent chromium.
 6. In a process of claim 2 wherein the catalyst comprises about 9 atomic percent chromium. 